Jewish Holocaust charities and Anne Frank's only living relative have criticised a new novel which suggests that Anne had a sexual relationship with the boy who was in hiding with her.

But the book's publisher, Klaus Flugge, hit back at critics of Sharon Dogar's Annexed, an imaginary diary by Peter van Pels which relates some of Anne's reflections, but from the boy's point of view.

Mr Flugge, as Ms Dogar's representative, said the author had communicated closely with Anne's cousin Buddy Elias throughout the drafts of her book. "She fully respects his role as the guardian of Anne's memory."

But the executive director of the Anne Frank Trust, Gillian Walnes, said she had been in touch with Mr Elias, Anne's only living relative, who, she said, was "very upset" about the book and had described it as "terrible".

Mr Flugge said: "He did not endorse the book, but he did not reject it either."

But asked whether he might have been uncomfortable with the suggetion that the teenagers were in love, Mr Flugge said that Mr Elias "is of a different generation, and they have different views."

Ms Walnes said that the book, due to be published in September by Andersen Press, was "cashing in" on the young diarist, and claimed that the author had never contacted the Trust.

She said: "We have to be scrupulous with the truth when it comes to Anne Frank. Anything else feeds the Holocaust deniers. It is so important."

She added: "This author is introducing the modern obsession with sexualisation of young people into a time when such things simply didn't happen - especially in such close proximity to the family. A 15-year-old girl in the 1940s would have been shocked at the thought."

Karen Pollock, chief executive of the Holocaust Educational Trust, also criticised attempting to fictionalise Anne's life. She said: "It risks trivialising what happened to Anne, her family and millions of other victims during this dark period of history."

The book, which will be translated into Hebrew and published in Israel, is the third novel from award-winning British author Ms Dogar, who writes "coming-of-age" books for teenagers.

Mr Flugge maintained the novel contained no sex scenes and that it was "categorically not an attempt to 'sex up' her incredibly important story".

He urged people to read the book before making a judgment and said that the author enjoyed a good relationship with Mr Elias.

He added: "Sharon is a thoughtful, sensitive and respectful author who has sought to increase awareness of the horrors of the Holocaust for a younger generation."

Anne, who died aged 15 in Bergen-Belsen concentration camp, wrote in her diaries about her feelings for Peter, who was one year older, and the two shared their first kiss.

On March 3, 1944, she wrote in her diary: "The boy has such warmth in his eyes; I believe I'm pretty near to being in love with him."

Later she wrote: "Remember yesterday's date, for it is a very important day in my life. Surely it is a great day for every girl when she receives her first kiss? Well, then, it is just as important for me too!"